The hearing impaired often struggle in complex acoustic environments (e.g., cocktail parties) where good localization abilities are particularly important. Many hearing-impaired people use only one assistive hearing device (e.g., hearing aid or cochlear implant), effectively limiting them to monaural (single ear) listening. Understanding the limitations monaural listening imposes on sound localization is of particular importance when evaluating the differences (e.g., costs, risks, and benefits) between monolateral and bilateral assistive devices. We hypothesize that listeners can discern the location of sound sources based on information in the monaural spectral profile. This hypothesis is tested in a series of psychophysical experiments that integrate the techniques of monaural studies of spectral profile analysis and binaural studies of spatial hearing. The overall goal of this research is to understand how the variability in speech affects the monaural localization abilities of normal-hearing listeners. The proposed work begins with the simple stimuli typically used in studies of spectral profile analysis and systematically builds towards speech-like stimuli. Although primarily a study of monaural localization, understanding how the variability in speech influences our perceptions is also important for understanding studies of binaural spatial hearing, speech intelligibility, and cochlear implant processing. The studies outlined here will provide important baseline data for evaluating (1) the degree to which monolateral assistive devices preserve information for monaural localization and (2) the benefits of bilateral assistive devices. In addition, determining the factors that lead to good monaural localization will aid in the design of better assistive devices (both monolateral and bilateral). Relevance The findings of this study will aid in the design of monolateral assistive hearing devices that preserve information useful for localization thereby reducing the difficulties that hearing-impaired individuals have in the prototypical cocktail party. The findings will provide important baseline data so that the benefits of assistive devices can be more accurately assessed. Additionally, comparisons between the results of this proposed work and previous studies have the potential to highlight shortcomings of current assistive devices and provide a guide for the design of the next generation of technology. [unreadable] [unreadable] [unreadable] [unreadable]